Can you please help me with this stoichiometry problem?

Ah, stoichiometry. Have you ever done ratios before?

When you are given chemical reactions, all of the moles of reactant and product are proportional. This means that they're in ratio. Say we double the number of moles of water to 12 moles. How many moles do you think C2H6 will have? It'd be 4 moles, since just like water it was doubled since they're in proportion.

You can also think about that example above like a literal ratio. Since the original ratio is 2:6, it then turns into n:12, with n being the unknown number of moles of C2H6. Since ratios we know that ratios are always equal can be converted to fraction:

2/6 = n/12
Cross multiply:
24 = 6n
n = 4 moles

Makes sense? On to the actual question.

We're looking at the proportion between oxygen and carbon dioxide. Let's write that into a ratio:

7:4

Since there are seven moles of oxygen and four moles of carbon dioxide.

The number of moles of carbon dioxide is reduced to 1 mole. You could simply divide seven by 4 to find your answer (Since 4 was divided by 4 to make 1), but let's use the ratio method just in case questions in the future aren't this convenient:

7:4 becomes n:1

Convert to fraction:
7/4 = n/1

Cross multiply;
7 = 4n

Divide:
n = 7/4

Convert to decimal:
n = 1.75

The answer is Option C, 1.75 moles.

Let me know if you want me to explain anything I did here.
Good luck!
-T.B.

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